The synthesis of fine chemicals and pharmaceuticals has become increasingly more complicated often requiring multi-step reactions involving complicated catalyst systems, such as, e.g., expensive enzyme and organometallic-based catalyst systems. Consequently, there has been increased emphasis on the development of new and improved catalyst systems which have high activity and selectivity, are easily recovered from reaction solutions for subsequent reuse and will maintain their catalytic activity for a relatively extended period of time under desired reaction conditions.
One such catalyst system which has shown great industrial potential in the field of biocatalysis, for example, are based on enzymes. Enzymes are proteinaceous catalytic materials that often exhibit the advantages of catalyzing difficult or complexed reactions with great chemical specificity under relatively mild conditions. For these reasons, there is increased emphasis on the use of enzyme-based catalysts in the food and pharmaceutical industries on a commercial scale.
Enzymes are generally soluble making recovery of the enzyme for reuse difficult, if not impossible. In some cases, the processing conditions may destroy the enzyme. Where the enzyme is not destroyed, it may be necessary to destroy it, as in the case of some food products, where continued activity would have an unwanted effect. To avoid these problems, fixed or immobilized enzyme systems have been developed where the enzyme is bonded onto the surface of an inorganic support or carrier. Exemplary immobilized enzyme systems, and the methods for the preparation thereof, are disclosed in U.S. Pat. Nos. 4,384,045; 4,258,133; 5,998,183; and 5,405,766.
Other catalysts of interest include organometallic complexes, which are widely used in the synthesis of fine chemicals and pharmaceuticals. Organometallic complexes catalyze many important reactions, such as, for example, Heck-type reactions, Suzuki coupling reactions, animation of aromatic halides, and Grignard reactions. In most applications, organometallic complex catalysis is performed in the homogeneous mode, where separation and reuse of the catalyst is difficult. Often, organometallic complexes are very expensive, so that reuse and recovery is highly desirable. A considerable amount of research has been aimed at “heterogenizing” the homogeneous organometallic complex catalysts, so that recovery of the catalysts is simplified. See, e.g., Cornils et al., Applied Homogenous Catalysis with Organometallic Compounds (Volume 3) (Wiley-VHC, 2002); and D. E. DeVos et al., Chiral Catalyst Immobilization and Recycling (Wiley-VC, 2002).
It is well known that many catalytic species, e.g., proteins, bind very strongly, and sometimes irreversibly and non-selectively, to certain support materials, in particularly, inorganic oxide-based materials. Further, where the inorganic oxide support contains a functionality such as hydroxyl groups, in particularly, acidic hydroxyl groups, the support can suffer an even higher degree of non-selective binding of the catalytic species. That is, the catalytic species, e.g., an enzyme, can bind to the surface of a support in a non-selective fashion decreasing catalytic activity. Therefore, while catalytic functionality on the surface can be very selective for the desired catalysis, the unused regions of the surface are often non-selective. The net effect is to lower the activity of the catalyst composite.
Consequently, there exists a need for improved supported catalyst systems which prevents or minimizes problems of non-specific binding associated with known supported catalyst systems, allow for easy recovery of the catalyst system from reaction solutions for subsequent reuse and maintain high catalytic activity for an extended period of time.